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Chapter 3 Atoms and Elements
3.4 The Atom
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Dalton’s Atomic Theory
In Dalton’s Atomic Theory, atoms
• are tiny particles of matter.
• of an element are similar and different from other elements.
• of two or more different elements combine to form compounds.
• are rearranged to form new combinations in a chemical reaction.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
John Dalton (1766-1844)
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Subatomic Particles
Atoms contains subatomic particles,
• protons have a positive (+) charge.• electrons have a negative (-) charge.• like charges repel and unlike charges attract.
• neutrons are neutral.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
4
Rutherford’s Gold-Foil Experiment
In Rutherford’s gold-foil experiment, positively
charged particles
• were aimed at atoms of gold.
• mostly went straight through the atoms.
• were deflected only occasionally.
Conclusion:
There must be a small, dense, positively charged nucleus in the atom that deflects positive particles that come close.
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Rutherford’s Gold-Foil Experiment
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Ernest Rutherford Gold foil experiment. A beam of positively charged alpha particles hits the gold foil. Most particles passed straight, some slightly deflected and some deflected back. The reason for deflection were the positions of the nucleus.
Separation of alpha, beta, and gamma particles by applying an electric field.
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Ernest Rutherford (1871-1937)
Rutherford’s interpretation of the gold foil experiment done by Geiger and Marsden.
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Modern View of the Atom
For an atom, which always has no net electrical charge, the number of negatively charged electrons around the nucleus equals the number of positively charged protons in the nucleus.
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Structure of the Atom
An atom consists• of a nucleus that
contains protons and neutrons.
• of electrons in a large empty space around the nucleus.
Copyright © 2005 by Pearson Education, Inc.Publishing as Benjamin Cummings
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Atomic Mass Scale
On the atomic mass scale for subatomic particles,
• 1 atomic mass unit (amu) has a mass equal to 1/12 of the mass of the carbon-12 atom.
• a proton has a mass of about 1 (1.007) amu.
• a neutron has a mass of about 1 (1.008) amu.
• an electron has a very small mass, 0.000549 amu.
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Particles in the Atom
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Scientists have been able to obtain computer-enhanced images of the outer surface of atoms using the scanning tunneling microscope (STM) and the atomic force microscope (AFM).
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Learning Check
Identify each statement as describing a
1) proton 2) neutron 3) electron
A. found outside the nucleus
B. has a positive charge
C. is neutral
D. found in the nucleus
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Solution
Identify each statement as describing a
1) proton 2) neutron 3) electron
A. 3 found outside the nucleus
B. 1 has a positive charge
C. 2 is neutral
D. 1, 2 found in the nucleus